1. Field of the Invention
The present invention relates to balance filters and duplexers, and more particularly, to a balance filter having surface acoustic wave filters connected in parallel and a duplexer using such a balance filter.
2. Description of the Related Art
Recently, cellular phones have employed a receive system in which a received signal is of differential type (balance type) in order to restrain common mode noise in an RF circuit. Thus, the RF circuit needs RF devices of differential type (balance type).
Japanese Patent No. 3391347 discloses a balance filter (hereinafter referred to as first conventional art). FIG. 1 schematically shows the first conventional art. Referring to FIG. 1, a balance filter 30 includes an input surface acoustic wave filter 34 and an output surface acoustic wave filter 36, each of which filters is formed on a piezoelectric substrate 32 and is of multimode type. The input SAW filter 34 includes a pair of reflection electrodes R1, an input IDT2, an output IDT1 and an output IDT3. The IDT1, IDT2 and IDT3 are interposed between the pair of reflection electrodes R1. The output SAW filter 36 includes a pair of reflection electrodes R1, an output IDT5, an input IDT4 and an input IDT6. The IDT4, IDT5 and IDT6 are interposed between the pair of reflection electrodes R1. The input SAW filter 34 and the output SAW filter 36 are connected in series by a connection line 31 that connects the output IDT1 and the input IDT4, and a connection line 33 that connects the output IDT3 and the input IDT6. An input terminal 38 is connected to one of a pair of electrodes of the input IDT2, and the other is grounded. An output terminal 39 is connected to one of a pair of electrodes of the output IDT5 and the other is grounded.
Power is applied to the input terminal 38, multiple standing waves develop between the reflection electrodes R1 of the input SAW filter 34. The input IDT2, output IDT1 and output IDT3 are designed so that the output IDT1 and output IDT3 respectively detect standing waves that are in opposite phase, that is, 180° out of phase. Thus, electric signals transmitted through the connection lines 31 and 33 have opposite phases. That is, the opposite-phase signals are transmitted from the input SAW filter 34 to the output SAW filter 36. The output IDT5, the input IDT4 and input IDT6 are designed so that the opposite-phase signals transmitted to the output SAW filter 36 are output via a pair of output terminals 39 with the above phase relationship being kept. The balance filter shown in FIG. 1 has the above-mentioned balance function of outputting the opposite-phase signals via the pair of output terminals 39.
It is essential to transmit the opposite-phase signals from the input SAW filter 34 to the output SAW filter 36 for realizing the balance filter shown in FIG. 1. The balance filter thus configured has excellent amplitude and phase balance characteristics.
Japanese Patent Application Publication No. 2003-249842 (Document 2) discloses a SAW filter in which double mode SAW filters are connected in parallel. Japanese Patent Application Publication No. 2006-74202 discloses a multimode SAW filter in which multiple stages of IDTs are connected in series.
However, the balance filter of the first prior art does not have good power durability because all of input power is applied to the single input SAW filter 34. This problem may be solved by increasing the number of IDTs of the input SAW filter 34. However, this method may affect the pass band and may have difficulty in securing a satisfactory wide pass band.
The electric signals output from the balance filter formed on a chip are in opposite phase. However, in practice, when the chip is incorporated into a package, the electric signals output from the package may deviate from the opposite-phase relationship. In the first prior art, the input SAW filter 34 must be adjusted. However, this adjustment has difficulty because the input SAW filter outputs the signals that are in opposite phase.